Intra-vascular, e.g. intra-cardiac, ultrasonic imaging catheters are used to examine the vasculature of the body, the heart, and surrounding tissue and organs. When the vasculature and its structures are being examined, the target anatomy is generally very close to the acoustic aperture and only limited acoustic penetration is necessary. These applications call for a high frequency transducer to maximize resolution while relaxing penetration requirements. For an array transducer this means that the array transducer elements should have a small pitch, the center-to-center spacing of adjacent elements, to reduce grating lobes and resultant image clutter. With piezoceramic transducers the pitch is often limited by the dicing process. Micromachined ultrasonic transducer (MUT) arrays, however, can be made very small since they are made by semiconductor processes. Hence, smaller pitch values can generally be obtained when CMUT and other MUT devices are used for the transducer array. The small sizes of MUT devices are advantageous for intra-cardiac catheters, when the array must be fabricated on the tip of a catheter that is sized to be threaded through the cardiovascular system. But other catheter applications call for the imaging of more distant organs and structures. For these applications greater penetration is needed and frequencies can be lower than for near field objects. The greater penetration requires greater transmitted acoustic pressures which is best met by high density arrays. Higher density array elements improve the performance in both the near and far field. It is thus desirable to be able to space adjacent MUT elements very tightly to improve pitch requirements, energy requirements, sensitivity, and hence imaging performance. Intra-cardiac catheters provide a further challenge beyond their small size requirements, which is that the transducer arrays must generally be curved to wrap around the cylindrical tip of the catheter. Such transducer arrays are sometimes referred to as sideways looking arrays.
EP 2 455 133 A1 discloses in FIG. 7 a catheter comprising such a sideways looking arrangement of CMUT arrays, wherein each array is separated by a neighboring array by a strip of electrical connections that facilitates the wrapping of the arrangement around the catheter. Although such arrangements can be effectively used in high-intensity focused ultrasound applications in which surrounding tissue may be treated with ultrasound, such arrangements are less suitable for imaging applications in which the discontinuity between neighboring arrays negatively impacts the imaging quality and resolution. Moreover, the CMUT arrays are typically mounted on rectangular rigid silicon islands.